/Engine/Source/Developer/TargetPlatform/Public/Interfaces/IShaderFormat.h
Virtual function that can be implemented by backends which require secondary compilation to specify a unique "key" identifier for the packing scheme (for validation in the runtime at load). see RequiresSecondaryCompile and the overloads of CompilePreprocessedShader for additional details.
virtual int32 GetPackedShaderKey() const